1. Field of the Invention
This invention is concerned with a way of eliminating distortion in a tapered-waveguide display of the kind in which light enters the blunt end of a tapered slab waveguide.
2. General Background of the Invention
Such a tapered-waveguide display comprises a video projector, a flat slab waveguide, known as an “input” or “expansion” waveguide, for spreading light from the projector in the width direction, and a tapered sheet of transparent material acting as a leaking waveguide, light exiting from its face at a point determined by the angle at which it entered. The taper starts at the same thickness as the expansion waveguide and decreases usually to zero. Light entering the taper at some angle up to the critical angle bounces ever more steeply until it exits at a distance along the taper that depends on the initial angle of injection. The system is described in the applicant's earlier application WO 01/72037.
These displays are cheap and robust because the screens need no thin-film transistors and can be made of plastic, but the projected image can be prone to distortion. In particular, the image may separate into bands with dark gaps in between, because there are certain points on the tapered exit waveguide that cannot be reached at the critical angle by any ray entering the taper. Thus between the nth bounce and the (n+1)th no light emerges. It is desirable to address this problem.